Retire the legacy scenario-dataset documentation page

Parent

Spec: #2454 — Document and teach the OEKG REST API. Derived from WF-14 decision D3, carried forward as spec decision D10 and note F1.

What to build

The documentation stops carrying a page for a route the API has replaced, and stops promising group-based shared editing, which was ruled out of scope and does not exist. The databus registration prose on that page is the only explanation of databus registration anywhere in the documentation, so it moves rather than being lost.

Why this is its own ticket: the same deletion was placed on the Study reports and dataset links pull request, which merged without it. The page is still published, still promising the feature, and its navigation entry is still indented one level too shallow — which is why it renders as a sibling of whole sections rather than as a child of the OEKG API. The condition that deferred it (the route not yet superseded) is now met.

Acceptance criteria

• The legacy scenario-dataset page is deleted.
• Its navigation entry is removed; the stray indent goes with it, so nothing is left rendering at the wrong level.
• The databus and external-URL prose is migrated into the client rules page.
• No remaining link in the documentation points at the deleted page.
• Changelog entry.

Blocked by

• #2460 — the client rules page — the databus prose migrates into that page, so it must exist.

Locate the legacy scenario-dataset page, its navigation entry, the client rules page, and the documentation changelog. Start by checking the page and navigation references, then migrate the databus and external-URL prose into the client rules page. Done means the page and links are gone, the navigation hierarchy is corrected, and a changelog entry is added.